The victims of mysterious attacks on workers at the United States embassy in Havana, Cuba have “developed changes to the white matter tracts that let different parts of the brain communicate,” per the AP. It’s the most specific physical symptom identified amongst the victims as doctors have begun to treat their condition as a previously unforeseen illness.

Why it matters: The finding provides some clarity that something tangible happened to the 24 embassy workers and family members who mysteriously fell ill in Havana. The U.S. government had previously blamed the attacks on a “sonic weapon” — though these new physical findings are not consistent with an acoustic attack — but has since avoided using that term.

Doctors treating the U.S. Embassy victims of mysterious, invisible attacks in Cuba have discovered brain abnormalities as they search for clues to explain the hearing, vision, balance and memory damage, The Associated Press has learned.

It’s the most specific finding to date about physical damage, showing that whatever it was that harmed the Americans, it led to perceptible changes in their brains. The finding is also one of several factors fueling growing skepticism that some kind of sonic weapon was involved.

These findings only add to the increasing worry about what weapon was used to attack the embassy and who was behind the attack:

Physicians, FBI investigators and U.S. intelligence agencies have spent months trying to piece together the puzzle in Havana, where the U.S. says 24 U.S. government officials and spouses fell ill starting last year in homes and later in some hotels. Secretary of State Rex Tillerson said Wednesday he’s “convinced these were targeted attacks,” but the U.S. doesn’t know who’s behind them. A few Canadian Embassy staffers also got sick.

Doctors still don’t know how victims ended up with the white matter changes, nor how exactly those changes might relate to their symptoms. U.S. officials wouldn’t say whether the changes were found in all 24 patients.

Here’s more from the report:

The case has plunged the U.S. medical community into uncharted territory. Physicians are treating the symptoms like a new, never-seen-before illness. After extensive testing and trial therapies, they’re developing the first protocols to screen cases and identify the best treatments — even as the FBI investigation struggles to identify a culprit, method and motive.

Doctors treating the victims wouldn’t speak to the AP, yet their findings are expected to be discussed in an article being submitted to the Journal of the American Medical Association, U.S. officials said. Physicians at the University of Miami and the University of Pennsylvania who have treated the Cuba victims are writing it, with input from the State Department’s medical unit and other government doctors.

But the article won’t speculate about what technology might have harmed the workers or who would have wanted to target Americans in Cuba. If investigators are any closer to solving those questions, their findings won’t be made public.